1. Field of the Invention
The present invention relates generally to nuclear fuel rod manufacture and, more particularly, is concerned with apparatus and method for welding an end plug on a nuclear fuel rod cladding tube.
2. Description of the Prior Art
A nuclear reactor core is composed of a multiplicity of fuel assemblies with each fuel assembly comprised of a plurality of nuclear fuel rods. Conventional operations performed at various stages in the manufacture of nuclear fuel rods are disclosed in a British Patent Specification No. to Foster (896,826) and in U.S. patents to Boyko et al (U.S. Pat. No. 3,683,148), Fink et al (U.S. Pat. No. 3,725,635), Heer et al (U.S. Pat. No. 3,774,010), Silk et al (U.S. Pat. No. 3,828,518), Duncan et al (U.S. Pat. No. 4,075,454), Yeo (U.S. Pat. Nos. 4,188,521 and 4,511,075), Christiansen et al (U.S. Pat. No. 4,480,171) and Miwa (U.S. Pat. No. 4,570,051).
According to one conventional method, a typical nuclear fuel rod is manufactured by loading a plurality of nuclear fuel pellets and a plenum spring into a cladding tube and applying bottom and top end plugs to opposite ends of the cladding tube. Typically, the bottom end plug is girth welded to one end of the tube at one welding station and then the tube is moved to a separate welding station where the top end plug is girth welded to the opposite end of the tube.
At each girth welding station, the respective tube end fitted with th corresponding end plug is inserted into a welding position in a welding chamber of a housing where the end plug engages a rotatable stop member and a circumferential seam or interface formed by the respective abutting shoulder of the end plug and end of the tube lies directly beneath a stationary welding electrode oriented orthogonally to the axis of the tube.
To obtain girth welding of the end plug in the tube, the tube and end plug and the stop member engaged therewith are rotated relative to the stationary electrode. Concurrently with the rotation, electric power is supplied to the stationary electrode, generating an arc which forms the girth weld at the circumferential interface of the end plug and cladding tube.
Ordinarily, the top end plug also contains an axial passage or bore. At still another separate station, a seal weld will be formed to close the axial bore of the top end plug to complete manufacture of the fuel rod. However, before formation of the seal weld, a vacuum is drawn through the end plug bore via a hole in a stop member engaged with the end plug. The vacuum evacuates undesired air and other contaminant gases from the plugged tube. This is followed by pressurized filling of the tube with an inert fill gas, such as helium, through the stop member hole and end plug axial bore. After that, an axial electrode is used to form the end seal weld to close the top end plug axial bore.
The above-described conventional fuel rod manufacturing method has several drawbacks. First of all, it uses separate welding stations for formation of each of the three welds. This requires duplication of expensive equipment and thus higher capital costs. Second, in each separate station, the welding chamber is flooded with inert gas to purge unwanted gases therefrom before the welding operation is carried out. This is highly wasteful of inert gas supplies. Finally, employment of separate welding stations makes the overall steps in carrying out fuel rod manufacturing more time consuming and cumbersome.
Consequently, a need exists for improvements which will eliminate or reduce the aforementioned drawbacks and enhance the efficiency of fuel rod manufacture.